1. Field of the Invention
The present invention relates to a hydrostatic stepless transmission (hereinafter, referred to as “HST”) including a hydraulic pump, a hydraulic motor and a closed fluid circuit fluidly connecting the hydraulic pump to the hydraulic motor. Especially, the present invention relates to the configuration and arrangement of a center casing constituting the closed fluid circuit and supporting plunger blocks of the hydraulic pump and motor, and relates to configuration for forcibly supplementing fluid to the closed fluid circuit in correspondence to improvement of the configuration and arrangement of the center casing.
2. Related Art
In a conventional HST as disclosed in JP 2008-281180 A, a pump plunger block rotatably integrated with a pump shaft has a flat surface at which plunger ports in the pump plunger block are open, a motor plunger block rotatably integrated with a motor shaft has a flat surface at which plunger ports in the motor plunger block are open, and the flat surfaces of the pump and motor plunger blocks contact a flat surface of a common plate-shaped or block-shaped center casing (or a fluid duct member) so as to be slidable and rotatable relative to the center casing. Hereinafter, the flat surfaces of the pump and motor plunger blocks and the flat surface of the center casing, which relatively slidably contact each other, are referred to as “slide surfaces”. The center casing is formed therein with a pair of main fluid passages constituting a closed fluid circuit fluidly connecting the pump plunger block to the motor plunger block. These main passages are open at the slide surface of the center casing so as to serve as an encased pump port and an encased motor port. When the slide surfaces of the pump and motor plunger blocks contact the slide surface of the center casing, the plunger ports in the pump plunger block are fluidly connected to the main fluid passages via the encased pump port, and the plunger ports in the motor plunger block are fluidly connected to the main fluid passages via the encased motor port.
The HST configured as mentioned above is often activated at a high speed or under a heavy load. During such a high speed or loaded activation of the HST, a problem causes that the pressure of hydraulic fluid flowing between the encased ports and the plunger ports becomes excessive so as to forcibly open a gap between the slide surfaces of the plunger blocks and the slide surface of the center casing, thereby leaking the fluid from the forcibly opened gap.
Further, the encased ports are normally kidney-shaped. The sizes of the kidney-shaped ports are limited by the size of the slide surface of the plunger blocks. As a result, total areas of the main passages in the center casing cannot be satisfactorily large, whereby the flow amount of fluid circulating in the main passages is reduced so as to reduce a speed of the flow, so as to increase a loss of hydraulic pressure of the fluid, and so as to cause noises. Especially, in a low temperature, the viscosity of hydraulic fluid may become excessively high so that the fluid cannot flow between the pump and motor plunger blocks, thereby hindering start of the HST.
Further, the area of the remaining external surface of the conventional center casing other than that contacting the plunger blocks is too small to be subjected to an enough air-cooling effect. In this situation, when the HST is operated at a high speed or under a heavy load, the hydraulic fluid is heated so as to lower its viscosity, thereby enhancing the leak of fluid.
Further, the shape and position of the conventional center casing are limited because it has to contact the slide surfaces of the plunger blocks. Therefore, the freedom of the positional relation between the pump shaft and the motor shaft is limited so as to limit the input and output directions of the HST. As a result, a design of an apparatus assembled with the HST is limited.
Further, each of the pump and motor shafts is stably supported at one end thereof via a bearing by a housing incorporating the hydraulic pump and motor and at the other end thereof via another bearing by the center casing. Such a configuration of supporting the pump and motor shafts increases the number of bearings and increases the size of the HST. Especially, when the pump and motor shafts are extended coaxially to each other, the HST becomes very long.